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Fixes module pinning for Forge modules. Per gds/puppet#953

I suspect that this will fix the following error, almost certainly caused by
pulling in a different version of the apt module:
```
Warning: Scope(Class[Apt::Update]): Could not look up qualified variable 'apt::update_timeout'; class apt has not been evaluated
```
Merge pull request #22 from alphagov/librarian_puppet_maestrodev

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